The disclosure relates to a system and process for characterization of semiconductor lasers, and more particularly for characterization of external cavity semiconductor lasers, and more particularly for automated testing and characterization of each external cavity semiconductor laser chips in a bar of laser chips prior to separation and mounting of each individual chip from the bar.
A quantum cascade laser (QCL) is a type of unipolar semiconductor laser that emits light primarily in mid-infrared (MIR) and far-infrared (FIR) wavelength range, e.g. in a range of wavelengths from about 3 μm to about 15 μm. An external cavity quantum cascade laser (EC-QCL) is a laser system combining a quantum cascade gain block (e.g. a laser chip with antireflection coatings on one facet) and an external cavity. The external cavity typically includes a collimation lens and a diffraction grating mirror (or simply diffraction grating). In a common Littrow configuration for a tunable external cavity semiconductor diode laser, the light emitted from the quantum cascade gain block is reflected by the diffraction grating at the first order diffraction back along the original beam path and back into the quantum cascade gain block to achieve lasing. Such a laser system is typically carefully designed so that it can lase at a single wavelength, which is determined by the grating angle (or Littrow angle) of the grating mirror. The grating angle is the angle between an axis extending normal to the diffraction grating and the axis of the path of the beam of light emitted from the quantum cascade gain block. When the diffraction grating rotates or pivots, the grating angle changes, and the lasing wavelength of the laser beam produced by the EC-QCL also changes. The lasing wavelength of an EC-QCL laser system can therefore be tuned within a certain range by rotating the grating mirror. The range within which the lasing wavelength can be tuned is determined by parameters such as the gain profile of the quantum cascade gain block, the anti-reflective (AR) coating on the facet of the gain block, the coupling efficiency between the gain block and external cavity, and the reflectivity of the first order diffraction of the grating mirror.
EC-QCLs provide a relatively wide spectral tunability of a single frequency mid-IR radiation, which finds numerous applications in mid-IR spectroscopy and molecular sensing. Characterization and optimization of the EC-QCL gain media parameters, such as characterization of the tuning range and electrical properties of the laser, play one of the most important roles in the EC-QCL development and design. Traditional methods for fabrication, characterization and selection of QCL gain media in the EC-QCL configuration requires several stages: 1) design, growth, and processing of the wafers, 2) wafer cleaving and separation into single EC-QCLs chips, 3) chip die bonding/mounting, 4) chip facet coating (one at a time), 5) individual EC-QCL chip testing, characterization and optimum gain media selection. While stage #1 is usually optimized to the best industrial standards, the processes of stage #2 to stage #5 are typically performed manually and are difficult to implement in high-volume industrial manufacturing and research and development environments. The separating, mounting, testing and characterization of each individual EC-QCL chip requires a significant amount of time and a well qualified operator to perform the complex laser-cavity alignment. Characterization of the laser chips is therefore a costly, time consuming and labor intensive process. There is a need for a process and system for testing and characterizing EC-QCL and other types of external cavity (EC) laser chips at the earliest stage of fabrications, such as prior to one or more of stage #2 through stage #5 described above, that eliminates wasted time and effort spent in stages #2 to stage #5 and provides a more efficient and cost-effective laser testing and characterization process.